1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to disk drives that are configured to compensate for disk eccentricity.
2. Description of the Prior Art
Disk drives, to varying extents, may suffer from a condition in which the disk or disks are not mounted at the exact center of rotation of the spindle motor. This condition manifests itself as a certain degree of eccentricity as the disk is rotated by the spindle motor. Such eccentricity may be caused by a condition called “disk slip” in which the disks slip relative to the clamp and/or spacers mounting the disks onto the spindle motor, which may occur as a result of a shock event occasioned by dropping or jarring the drive, for example. Alternatively, disk eccentricity may be caused, for example, by an inexact installation of a pre-recorded disk or disks (from a media writer, for example) on the drive's spindle motor. Some eccentricity is unavoidable, as the center opening of the disk must, by definition, be larger than the spindle of the spindle motor onto which it must be fitted.
When a disk or disks rotate with eccentricity, the result is once per revolution (OPR) timing errors, also referred to as a “big run-out”. In contemporary drives, servo sectors of servo information are interspersed with data sectors circumferentially in concentric tracks around the recording surface or surfaces of the disk or disks. The format of the tracks calls for regularly spaced embedded servo sectors (also called servo wedges) containing servo information therein. Between the embedded servo sectors are a number of data sectors, which are configured to store user data. When a disk rotates with eccentricity, the wedge-to-wedge timing (as determined by the timing of circumferentially adjacent Servo Sync Mark (SSM) signals) will be modulated by the OPR timing error as the disk rotates, with the timing error being worse toward the inner diameter (ID) of the disk. This variation in the wedge-to-wedge timing due to disk eccentricity manifests itself as a certain degree of variation in the apparent linear velocity of the disk relative to the read/write transducers and causes a certain degree of uncertainty in the timing of the servo control signals and the placement of user data. This timing uncertainty must be accounted for in the allocation of the various fields and the gaps interspersed between the data fields, which results in a loss of format efficiency, as compared to a situation in which such timing uncertainty were minimized.
In order to develop a tighter speed control, the time interval between successive circumferentially adjacent SSMs may be measured to determine the “correctness” of the velocity of the disk (in counts per wedge). By feeding a clock to a counter, such as a wedge-to-wedge timer and comparing this feedback to an expected value, the resulting error may be taken as an indication of the necessary correction. However, such a method is not optimal in the presence of eccentricity, when the SSMs timing intervals are modulated by OPR timing errors caused by the changes to the apparent linear velocity of the disk for different areas of the track.
From the foregoing, it may be appreciated that methods for reducing the effects of disk eccentricity are needed.